Method of fabricating cathode ray tube screen

ABSTRACT

A method for providing a tri-dot patterned color cathode ray tube screen having phosphor patterns of improved symmetry. The photofabricating method involves discretely refracting light for exposing a sensitized screen panel through an associated apertured mask by utilizing a predeterminately positioned optical system. A portion of the exposure light radiation is refracted therein by a modified plano-concave lens having at least one modified plano chordal portion whereby improved exposure is provided for one or more specific peripheral quadrantal portions of the screen to effect substantially improved equilateral phosphor dot pattern formation thereon.

United States Patent Inventor Glen A. Burdick Waterloo, N.Y.

Appl. Nov 851,381

Filed Aug. 19, 1969 Division of Ser. No. 605,783, Dec. 29, 1966, Pat. No. 3,509,802

Patented May 25, 1971 Assignee Sylvania Electric Products Inc.

METHOD OF FABRICATING CATHODE RAY TUBE SCREEN 5 Claims, 14 Drawing Figs.

US. Cl 29/2513, 95/1 Int. Cl H01j 9/18 Field of Search 29/2513, 25.14, 25.15, 25.17,25.11; 95/1 References Cited UN lTED STATES PATENTS 1,245,606 11/1917 MacCurdy 95/1 2,817,276 12/1957 Epstein 95/1 2,885,935 5/1959 Epstein 95/1 2,936,683 5/1960 Burdick et al. 95/1 2,986,080 5/1961 Burdick 95/1 Primary Examiner-John F. Campbell Assistant Examiner-Donald P. Rooney Attorneys-Norman J. OMalley, Donald R. Castle and Frederick H. Rinn ABSTRACT: A method for providing a tri-dot patterned color cathode ray tube screen having phosphor patterns of improved symmetry. The photofabricating method involves discretely refracting light for exposing a sensitized screen panel through an associated apertured mask by utilizing a predeterminately positioned optical system. A portion of the exposure light radiation is refracted therein by a modified plano-concave lens having at least one modified plano chordal portion whereby improved exposure is provided for one or more specific peripheral quadrantal portions of the screen to effect substantially improved equilateral phosphor dot pattern formation thereon.

PATENTED M12519?!" 3579.768

sum 1 0F 7 I c INVENTOR.

v GLEN A. BURDICK g; m E pmw 20 V v I ATTORNEY PATENIED was an SHEET 3 UF 7 INVENTOR GLEN A.BURD|CK ATTORNEY METHOD OF FABRICATING CATHODE RAY TUBE SCREEN CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION This invention relates to cathode-ray tubes and more particularly to improved patterned screens and a method of forming said improved screens for color cathode ray tubes.

Cathode-ray tubes employed as display devices in color television applications conventionally utilize one or more electron guns to provide the required electron beam or beams which are accelerated, focused and modulated by voltages applied to the gun. When a plurality of guns are used in a related manner, convergence electrodes or pole pieces are usually included as part of the electron gun structure. The modulated electron beams are discretely deflected across the screen to provide electron impingement upon selected color fluorescing materials disposed in patterned configurations on the viewing panel of the tube to reproduce the transmitted color image. It is conventional practice to position a grid or grids or an apertured mask intermediate the electron gun or guns and the cathodoluminescent screen to provide focusing and deflection of the electron beams or selected masking of the screen.

Color cathode-ray tubes, of the type generally utilized for color television, usually have screens consisting of multitudinous dot, bar or stripe patterned fonnations of green, blue, and red color fluorescing phosphors. Various methods have been employed to fonn therespective screens, for example, one conventional process makes use of a photoprinting technique. By this procedure, the tube viewing panel is coated with a light sensitive substance and one of the desired electron-responsive phosphor materials and then exposed to a point source of light through an appropriate negative master. Subsequent development produces a first phosphor pattern of desired configurations. The process is repeated for each of the remaining different color electron-responsive phosphors comprising the patterned screen. In each instance, the point source of light is appropriately offset during the exposure operation to provide individual phosphor patterns that are properly displaced from one another to form the desired screen.

One difficulty encountered with photodisposed screens is caused by the electrons not following the same beam paths during tube operation as the light rays travel during the screen forming procedures. Consequently, there are areas of the screen where the electron beams do not properly impinge the desired patterned configurations during tube operation. This condition of misregister results in the display image having color impurity.

The inherent nature of the electron beam is a factor affecting the aforementioned misregister. Since the electrons projected toward the screen have mass and charge, their paths of travel are altered by the electron gun and tube geometry and by various electrostatic and magnetic fields existing in and around the tube. It has been found that the center of deflection, or the location within the deflection yoke from which the electrons appear to come, moves as the electron beam scans the screen. In addition, when several electron guns are utilized to effect a plurality of electron beams the phenomena resulting from the dynamic convergence of the several beams causes a departure of the beams from their normal paths of travel.

Various procedures and devices for reducing the amount of misregister between the electron beam or beams and the fluorescent pattern configurations have been employed. Auxiliary devices have been used internally and externally of the tube and on and about the tube components in an effort to compensate for the excursion of the electron beams from the desired trajectories. In the screen exposure devices, several 2 light optical systems have been devised to consummate the photodeposition of the desired screen pattern. For example, in tubes having a discretely dotted screen pattern formed relative to a foraminous shadow mask wherein a triad of dots of differ ing cathodoluminescent phosphors are related to each aperture in the mask, several types of lens components have been incorporated into the light optical system. For instance, in color tubes having a 70 angle of deflection, it has been found that use of a spherical symmetrical planar-concave lens, properly tilted and offset, provides light optics which substantially duplicate the electron optics of the operating tube as effected by the axial motion of the center of deflection and dynamic convergence. Screen exposure systems of this type are described and claimed in U.S. Pat. No. 2,986,080 issued to Glen A. Burdick, and U.S. Pat. No. 2,936,683 issued to Glen A. Burdick et al., both of which are assigned to the same assignee as the present invention.

In the manufacture of screens for color tubes having angles of deflection greater than 70, for instance, rectangular tubes having deflection, it has been discovered that use of the aforementioned lens does not achieve the desired register relationship between light optics and electron optics. In at least two areas of the screen, in substantially the upper right and upper left-hand corner areas, the desired equilateral dot patterns are not fully realized. A related lack of symmetry exists in the 70 tube exposure optics, but due to the smaller angle of deflection the effect is not significant in magnitude, and the dot pattem-beamregistration is acceptable for all areas of the screen. For the most part, in the screen of the 90 tubes, the dot and beam patterns are substantially in desired registry for all parts of the screen except in the areas of'the two mentioned upper comer regions of the viewed screen. In the upper right comer areas the dots of one color tend to be displaced in an outward radial direction in their triad patterns, and in the upper left corner area dots of another color areaffected in a similar manner.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to reduce the aforementioned difficulties and to provide an improved color cathode-ray tube.

Another object is to provide a method of fabricating color cathode ray tube screen having a dot pattern of improved sym metry.

The foregoing objects are achieved in one aspect of the invention by the provision of an improved method for' photofabricating an improved color cathode ray tube screen having a patterned mask in spaced adjacency therewith. The light radiation for screen exposure is discretely refracted by a modified piano-concave lens having nonsymmetrical optical aberration positioned between a light source and the masked sensitized panel. Improved light refraction is achieved by utilizing the modified lens in a manner that the modified por-' tion of the piano surface formed as at least one substantially chordal area is oriented to provide desired correction of light ray refraction and direction to a discrete area of the screen'to thereupon effect the optical imprinting of a screen pattern having improved symmetry.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 5 is a top plan view looking down through the panel I into the apparatus;

FIG. 6 is a plan view showing the optically utilized portion of the refractive medium;

FIG. 7 is a sectional view of the refractive medium taken along the line 7-7 of FIG. 6; 1

FIG. 8 is a sectional view showing another embodiment of the invention;

FIG. 9 is a sectional view showing the transverse profile of the modified first chordal portion taken along the line 9-9 of FIG. 7;

FIGS. 10 and 11 are sectional views showing alternate embodiments of the transverse profile of the modified first chordal portion taken along the line 9-9 of FIG. 7;

FIG. 12 is a sectional view showing the transverse profile of the modified second chordal portion taken along the line 12-12 ofFIG. 8; and

FIGS. 13 and 14 are sectional views showing alternate embodiments of the transverse profile of the modified second chordal portion taken along the line 12-12 of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawings.

With reference to the drawings, FIG. 1 illustrates a conventional plural beam shadow mask type of color cathode-ray tube 11 having a central axis 12 therethrough. Suitably disposed within the neck portion of the envelope 13 are three electron emitters 15 oriented, for example, substantially l apart equally spaced about axis 12 to provide a delta arrangement of electron beams 17, 18 and 19. Coil means in the form of yoke 20 positioned external of the envelope, are utilized to deflect the electron beams over the raster area. It is desirable that the several beams converge at the mask 21 and pass through apertures 22 therein to discretely impinge the patterned cathodoluminescent screen 23 therebeneath. The screen comprises a multitude of triadically arranged dots or elements of green, blue and red color electron responsive fluorescing materials formed on the interior surface of the viewing panel 25. While a tri-gun shadow mask color tube is illustrated in FIG. 1, it is not intended to be limiting as the invention to be described in this specification is also applicable in other types of image reproduction devices employing plural beams of radiant energy excitation.

Since the tube axis, the panel axis and the electron gun system axis are substantially coincidental, it seems expedient for clarification in this description to henceforth denote these several reference axes as the central axis 12.

To achieve the desired convergence or crossover of the several beams at the mask apertures, external dynamic convergence means 27 are normally utilized. With referenceto FIG. 2, two of the three beams, 17 and 18, respectively, are shown to illustrate the aspects of dynamic convergence. The static convergence beam paths, 17 and 18, passing through points a and b, respectively, in the deflection region, proceed at an angle with the tube axis to converge or crossover at the apertured mask 21 and thence impinge the patterned screen 23. In deflecting the beams to an angle alpha (:1) without the influence of the dynamic convergence means 27, the beams appear to come from points a and b and undesirably efiect convergence short of the mask at point f. Employment of the dynamic convergence means provides magnetic fields which move the beam positions 17' and 18' radially outward in the deflection region to cause the beam positions 17' and 18' to appear to come from points 0 and d to provide the desired convergence crossover at the apertured mask. It will be observed that as the angle of deflection increases, the deflected electron beam appears to emerge from the deflection region at a point closer to the screen. For example, as the electron beam path 17 is deflected from the static position to the convergence deflected beam path 17, the dotted line between a and c defines the locus of motion of the apparent center of deflection. In like manner, movement of the. related beam from static beam path 18 to the convergence deflected path 18' defines the locus of motion of the apparent center of deflection as being along the dotted line between b and d. The apparent center of deflection and locus of motion thereof is difierent for each electron beam because of the respective electron gun orientation within the tube.

Screen 23 is comprised of multitudinous triadical groupings of electron responsive phosphor dots 35, 36, and 37 respectively. Two of such triadical groupings are illustrated in FIG. 2, namely, an axial grouping 31 and a radial grouping 33. The geometry of the tube is such that the three electron beam landings form a substantially equilateral triadical formation at the center of the screen and a radially compressed triadical formation at the peripheral region thereof. With the optical system conventionally utilized for screen exposure, the photodisposed triadical phosphor dot patterns, while substantially equilateral over a major portion of the screen area, depart significantly from equilaterality in certain peripheral areas. This departure is particularly noticeable in tubes having wide angles of deflection in excess of 70, as for example deflection in rectangular screen tubes. Such is shown in FIG. 3 wherein fragmentary portion of a prior art rectangular screen 39 is portrayed from the viewpoint of an observer facing the viewing panel 25. Illustrative groupings of dots and beam landings are shown in exaggerated size. The axial dot grouping 31 has substantially equilateral placement of phosphor dots 35, 36, and 37. When the electron beams 17, 18, and 19 pass through the mask and make landings on the electron-responsive dots, the areas of impingement fluoresce in a color characteristic of the particular phosphor, such as green (G), red (R), and blue (B), respectively. In the upper left screen region 41, the illustrated radial grouping 33 shows an outward radial displacement of the phosphor 45 dot with reference to adjacent dots 46 and 47. The criticalness of this displacement is evidenced when the deflected beam 17' makes a peripheral G landing thereon. A similar situation exists in the upper right screen region 51 where an exemplary upper right radial triad 43 has the red phosphor dot 56 radially displaced.

It has been found that correction of the misplaced dots in the aforementioned comer regions of rectangular screens can be achieved by utilizing the method and the specialized optical system of the invention during screen exposure.

An optical exposure apparatus, such as that depicted in FIG. 4, is utilized in the method to fonn the aforementioned dot patterned screen. It is desired to discretely dispose the triadical dot patterns in a manner that the subsequent electron beam landings will be in register therewith and have the largest possible minimum border of fluorescent material around each beam impinging position. Prior to the exposure of each of the several patterns comprising the screen, the screen-bearing surface, in this instance the inner surface of the viewing panel 25, is coated with a light hardenable photosensitive substance and a desired electron responsive color cathodoluminescent phosphor material, one for example being zinc-cadmium sulfide which fluoresces green, to form a photosensitive phosphor-associated film 61 thereover. Next, the apertured mask is temporarily positioned in spaced adjacency with the sensitized panel, whereupon the mated mask-panel assembly is suitably oriented on the exposure apparatus 65. Within this apparatus, there are means 67 for predeterminately positioning an optical system 68 comprising a point light source 69 and a light refractive medium in the form of a modified planoconcave lens 71 wherein a portion of the piano surface is distinctly modified. In the exposure step, discrete areas of the coated panel are subjected to light radiating from the point light source 69 which is refracted in a predetermined manner by the lens 71 and directed through the mask apertures 22.

The discrete areas of the photosensitive film 61 which are exposed to the light radiation become hardened and adhere to the surface of the glass panel forming an imprint of a first screen pattern of dots. Sequentially, a screen developing step removes the intervening unexposed portions of the film shadowed by the solid portions of the mask structure wherein the panel is treated with a suitable solvent or developing fluid. The above-described procedure is twice repeated to dispose the required blue and red phosphor dot patterns of the complete screen combination. For the separate exposure of each screen pattern, the light source and lens are properly positioned and offset from the central axis, the optical system being shifted substantially 120 about the central axis for each subsequent pattern exposure.

With reference to FIGS. 4, 5, 6, and 7 the essentials of the optical system 68 are shown in greater detail wherein a light refractive medium such as a modified plano-concave lens 71 of high UV transmissive optical glass is positioned intermediate the point light source 69 and the apertured shadow mask 21. As previously described, the locus of motion of the apparent center of deflection in the operating tube appears to move forward toward the screen as the angle of deflection increases. In a like manner the apparent origin of the light source appears to follow a similar locus due to the optical aberrations designed into the lens. Of the light emanating from source 69, a ray 73 designated as a single line is selected for illustration. Upon being incident upon and refracted by lens 71, subject ray is directed to a particular portion of the mask 21 to pass through an aperture therein and light expose a phosphor dot area 75 of the screen. The electron beam for the same deflection angle appears to originate at point M, which also appears to be the apparent light source for ray 73. Another light ray 77, directed to an opposite portion of the screen to expose phosphor dot area 79 is likewise refracted to have an apparent light source at M. With N designating the apparent light source for a ray 81 beamed to the center dot area 83, the apparent locus 86 of motion of the light source is along the line M-N. Although the light rays 73 and 77 originate at the point source 69, they appear to come from a point on locus 86, when viewed from the screen, due to refraction of the modified lens 71. Thus, (with reference to FIGS. 2 and 4) the optical system of the exposure device as utilized in the method of the invention produces the desired spacial relationship between the respective loci of motion 85 and 86 of the apparent center of electron beam deflection and the loci of the apparent origins of the light beams to effect the desired register between the phosphor dot screen pattern and respective electron beam impingements thereon. The optical system illustrated is, for example, capable of photodisposing the green fluorescing screen pattern whereof a dot in substantially the upper left comer or 10 o'clock area of the viewed screen is designated as 75 and one at substantially the lower right corner or 4 o'clock area thereof as 79. The relationship of subsequent electron beam impingement on these respective phosphor dots is indicated in FIG. 5 as 76 and 76.

The design of the modified plano-concave lens and the positioning of the lens relative to the masked panel, the central axis and the light source may be adequately achieved in several ways: by mathematical calculation, by empirical experimentation, or by a judicious combination of the two.

The modified plano-concave lens 71 as utilized in the improved optical system of the invention for photodisposing the screen of a 25-inch rectangular shadow mask tube having substantially 90 deflection, has corrective qualities for optical aberration substantially equivalent to the length of the locus of motion of the apparent center of deflection for all angles included within the subject 90 deflection. By way of illustration, the basic circular lens 70 has a symmetrical concave portion or spherical concavity 89 having a radius of curvature 84 of approximately 83.500 inches determined from a point 87 on the basic lens axis 91 which is also referred to as the quasiaxis of symmetry. The thickness of the lens p at this quasi-axis is approximately 0.200 inches. In this example the lens plano portion 93 has a modified chordal section 95 shaped as a substantially circular cylindrical section with the surface 97 thereof transitionally tangent to the plano surface along a substantially linear region ofdemarcation 96. The axis 98 of the substantially cylindrical section is spacedly oriented from the concave surface 89 in a plane 100 substantially parallel to the plano lens surface 93, the axis 98 of the cylindrical section being substantially parallel to the substantially linear region of demarcation 96 and in a plane 102 therewith perpendicular to the plano surface 93. The radius 94 of the substantially cylindrical surface, as determined from the axis 98, is in the order of 85.00 inches. Thus, a slight convex curvature is imparted to the chordal area in a direction toward the spherical concavity which effects a gradual reduction in the refractive thickness of the lens. It has been found that this chordal modification improved the refraction of the light rays directed to the upper left region of the screen and brings the ray landings radially inward for desirably disposing the green phosphor dots.

It has been found that the orientation of the optical system to produce the desired aforedescribed dot placement in the 25-inch rectangular panel can be accomplished by orienting the several axes of related elements of the system in a common vertical plane. For example, there is contained in subject common plane the central axis 12 wherefrom the axis 72 of the point light source 69 is laterally offset by a distance k of approximately 0.180 inches; and wherein there is also located the quasi-axis 91 of the lens component 71 which is offset from the central axis by a distance p of approximately 1.938 inches. Optimizing of the angles of incidence and refraction is effected by imparting a slight tilt to the lens which is achieved by tilting the quasi-axis thereof at an angle (B) of substantially 05 from a line 14 parallel with the central axis 12 and oriented in the common vertical plane therewith. Consummation of the lens tilt is accomplished about a lateral axis of placement 92 which perpendicularly intersects the quasi-axis 91 and the lens plane of symmetry 99. The amount of offset and tilt of the modified lens 71 are interrelated in a compensating manner whereof an increase in offset will permit a reduction in tilt and vice versa to achieve the desired results.

It will be noted that the basic lens 70 is offset in the exposure device in a manner that only a portion of the spherical concavity 89, removed or shifted eccentrically from said quasi-axis, is optically utilized as a nonsymmetrical spherical concavity. The concavity in conjunction with a portion of the plano surface 93 and the chordal area 95 constitute the elements of utilized light refractive medium 71. In addition, it will be noted that the quasi-axis is retained therein. The portion of the basic lens 70 designated by s is not optically utilized in the application, and if desired can be removed by forming a smaller optical unit therefrom embodied by the dimension t in the form of refractive medium 71' which contains theoptical essentials for the application previously designated by refractive portion 71 of basic lens 70. Henceforth, in this description, to enhance clarity, the utilized refractive medium will continue to be referred to as 71 and the extraneous portions of basic lens structure 70 will be disregarded. Due to the rectangular screen shape, differing portions of the refractive medium 71 are utilized to expose the dot pattern of each respective color producing phosphor.

With particular reference to FIG. 5, looking into the panel 25 positioned atop exposure apparatus 65, positioning planes utilized in orienting the optical system for photodisposing the several color fluorescing phosphor patterns are indicated. For example, the green dot pattern is formed by positioning the optical system in plane CD which substantially corresponds to the 4l0 o'clock panel diagonal, being removed clockwise from the 12 o'clock position in ordinate plane AB by The plane of symmetry 99 of refractive medium 71 is positioned coincident with plane CD. This plane of lens symmetry bisects the first chordal portion 95 in a manner perpendicular to said substantially cylindrical section and has the quasi-axis 91, the light source axis 72, and the central axis 12 contained therein. The light irradiation directed to the upper left or 10 o'clock quadrantal area 41 of the screen is refracted in an improved manner' by the chordal lens portion 95 to pull the light beam impingement radially toward the central axis 12. This refractive improvement photodisposes the green dots 75 in pattern exposure for the remaining area of the sensitized screen panel.

In photodisposing the red dot pattern the optical system is positioned along plane EF which substantially corresponds to the 2-8 oclock panel diagonal, being removed clockwise from the 12 oclock position by 240. In FIG. 5 while the whole optical system is shifted to coincide with plane EF only light source 69a is shown to avoid confusion in the drawing. In this position, the first chordal portion 95 effects refractive improvement for the red dot pattern in the upper right or 2 oclock quadrantal area 51 of the screen bringing the red dot positions 105 radially inward into an improved triadical position so that the subsequent electron beam impingement will be better centered thereon.

For blue dot deposition the optical system is oriented in the AB ordinate plane wherein light source 69b is indicated. With the optical system so positioned, the refractive improvement effected by the first chordal portion falls within an area 107 which is substantially outside of the rectangular panel and needs no correction, but in a round panel 106 correction of blue dot orientation would be desired in said area. Thus, in a rectangular screen the first chordal lens portion affords improved dot positioning for each of two color patterns in substantially a specific area of the screen for each. This facilitates a subsequent color display of improved color purity since the color dot patterns are more desirably positioned in these certain screen areas to be in better register with electron beam impingement. The various subsequent electron beam landings are designated in FIG. 5 to indicate the improvement produced.

Since the optical system 68 may be tilted with reference to the central axis 12 or the distance of offset of the lens or light source may be varied therefrom or the angle oflens tilt varied, it may be expedient to modify a second portion of the plano surface of the lens. A second embodiment of the invention as shown in FIG. 8 allows a change in the lens tilt-lens offset relationship wherein a second chordal portion 111 is modified to make correction to dot placement in the screen areas oriented substantially opposite the influence of the first chordal portion 95. The modified planoconcave lens 74, which has a usable refractive portion similar to that exhibited by refractive medium 71, as illustrated in cross section 'in FIG. 8, has a first modified chordal portion 95 with a substantially cylindrical surface 97 as previously described for the initial embodiment. In addition, there is a second modified chordal portion 111 substantially diametrically opposed to the first chordal portion 95 with an intervening substantially unmodified plano portion 93' therebetween; the second chordal portion being oriented substantially opposite the nonsymmetrical spherical concavity 89. This second chordal portion, substantially involving the section v, is formed as a second substantially cylindrical surface 113 transitionally tangent to the plano surface 93 at a second substantially linear region of demarcation 108 to impart a slightly opposed concave curvature in a direction away from said spherical concavity and effect a gradual increase in the refractive thickness of the light refractive medium 74 in a second selected peripheral area thereof. The axis 109 of the second substantially cylindrical section is spacedly oriented from the plano surface 93 in a plane 115 substantially parallel thereto, the axis 109 being substantially parallel to the linear region of demarcation 108 and in a plane 119 therewith perpendicular to the plano surface 93. The radius 117 of the second substantially cylindrical surface, as determined from axis 109, is of a value to effect the required arcuate surface for consummating the desired refraction. The plane of symmetry of this refractive medium bisects both chordal portions and is oriented in the optical system as for the previously described embodiment. The utilization of this second light refractive medium facilitates dot placement compensation in diagonally opposite areas of the screen when such is desired.

While the modified first and second chordal portions of the plano lens surface have been described as being substantially circular cylindrical sections, optical requirements may necessitate forming the chordal surfaces in accordance with sequentially differing radii 94 and/or 117 in effecting peripheral contours other than circular. If such be the case for either or both chordal surfaces, the planes and/or containing the respective axes 98 and 109 may be parallelly shifted in spaced relationship to the plano lens surface according to radial requirements. In addition, it may be desirable for optical considerations to effectuate a departure from cylindricity in all or a portion of each or both of the modified chordal surfaces. For example, with reference to FIGS. 7, 9, 10 and 11, the profile of the substantially cylindrical general surface 97 of the first chordal portion 95, as transversely defined by lines of intersection between planes parallel with the perpendicular plane 102 and the general chordal surface 97, may be a substantially straight surface as indicated by the substantially straight profile 121 as shown in the embodiment illustrated in FIG. 9. Optical requirements may necessitate the formation of a slightly curved surface, as for example, an arcuately ellipsoidal surface as characterized by the elliptic line 123 in the embodiment shown in FIG. 10, or an arcuately hyperboloidal surface as depicted by the hyperbolic line 125 in the embodiment illustrated in FIG. 11, or the profile surface may be a blended combination of the aforementioned surface embodiments. Similarly, with reference to FIGS. 8 12, 13 and 14, the profile of the substantially cylindrical general surface 113 of the second chordal portion 111 as transversely defined by lines of intersection between planes parallel with the perpendicular plane 119 and the general chordal surface 113 may be a substantially straight surface as indicated by the substantially straight profile 127 as illustrated in the embodiment shown in FIG. 12. As with the first chordal profile, a slightly curved surface may he optically advantageous for the second chordal portion, such as that portrayed in the embodiment in FIG. 13, wherein an arcuately ellipsoidal surface is indicated by the elliptic line 129, or an arcuately hyperboloidal surface as shown in the embodiment of FIG. 14 by the hyperbolic line 131, or a blended combination of the several surface embodiments. Thus, the term substantially cylindrical" as used in this specification with reference to either or both of the modified chordal lens portions is intended to encompass peripheral surface contours transitionally ranging from substantially right cylindrical to cylindrical manifestations and transverse profiles ranging from substantially straight to arcuate or respective combinations thereof.

Use of the improved method for photofabricating patterned screens has markedly improved color screen quality and has provided for the desired register of the dot patterns with electron beam impingement in tubes having deflections in excess of 70. Thus, a patterned cathodoluminescent screen is produced wherein the phosphor dots in substantially all portions of the screen are in contiguous abutment. This disposure arrangement forms an expansive substantially regular tessellation of dots which are discretely oriented as a multiplicity of similar triadical phosphor groupings. A screen so formed produces a vastly improved display having significantly enhanced color purity.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

lclaim:

1. A method of photofabricating at least one of a plurality of triadically related dot patterns forming the cathodoluminescent screen for a color cathode ray tube having a central axis, a screen-bearing panel, and a system of electron guns emanating plural electron beams capable of being deflected from static paths to scan said screen through an intermediately positioned apertured mask spaced adjacent to said screen, said central axis and the axis of said panel and said electron gun system being substantially coincidental, said method being consummated with reference to one of three equispaced orientation diagonals intersecting said central axis and traversing said panel, said method comprising the steps of:

applying a coating of a light sensitive substance and an electron responsive phosphor material to said screen-bearing panel; orienting said mask in spaced adjacency to said coated panel to provide a mated mask-panel assembly; positioning said mated assembly relative to a point source of light radiation having an axis laterally removed from said central axis, said light source being oriented relative to the subsequent positioning of one of said electron guns to discretely direct light radiation through said apertures of said mask to said sensitized panel; positioning a refractive medium in the fonn of a modified plano-concave lens in a predetermined manner intermediate said light source and said mask with said concave surface being oriented toward said mask, said lens having a plane of symmetry containing a quasi-axis of symmetry and a lateral axis of placement perpendicularly intersecting said quasi-axis and said plane of symmetry, said modified portion of said plano surface being at least one substantially chordal portion thereof bisected by said plane of symmetry, said lens positioning being in a manner wherein said plane of symmetry contains said light source axis and said central axis with said quasi-axis being removed from said central axis therein;

retracting said light radiation from said source through said lens in a manner to effect directional irradiation to said masked panel to provide a locus of motion of apparent light beam origin that is substantially coincident with the locus of motion of the apparent center of said subsequent electron beam deflection, a portion of said light incident upon said lens being refracted by said modified chordal portion whereof said light rays incident thereupon are directively refracted to a substantially quadrantal area of said mask;

exposing said coated panel through said mask to provide the selective imprinting of one of said plural screen patterns on said light sensitive substance; and

developing said selectively exposed panel to remove the unexposed material therefrom to provide one discrete dot pattern of said screen.

2. The method of fabricating a screen pattern according to claim 1 whereof said light refracting and exposing steps are dependent upon the positioning of said lens wherein said concave surface thereof being formed as a spherical concavity is positioned relative to said central and said light source axes in a manner that the concave lens portion substantially utilized in screen exposure is a section eccentrically removed from but containing said quasi-axis and defined as a nonsymmetrical spherical concavity, and wherein the positioning of said lens orients a first chordal portion thereof on the opposite side of said central axis from said quasi-axis to refract that portion of said light radiation incident thereon to substantially the adjacent quadrantal portion of said panel in a manner to move the light ray landings radially inward and provide the disposure of a substantially equilateral phosphor dot pattern thereover, said first chordal lens portion being shaped as a first substantially cylindrical section with the surface thereof tangent to said plano surface at a first substantially linear region of demarcation to impart a slight convex curvature to said modified surface in a direction toward said spherical concavity and effect a gradual reduction in the refractive thickness of said lens in a first selected substantially peripheral area thereof.

3. A method of fabricating a dot screen pattern according to claim 2 wherein the refracting of said light by the positioning of said modified lens is further defined by optimizing the angles of incidence and refraction thereof by tilting said lens on said lateral axis of placement relative to said central axis.

4. The method of fabricating a screen pattern according to claim 2 whereof said light refracting and exposing steps are accomplished by positioning said lens and said light source rela- -.tive to one another and to said central axis and one of said orientation diagonals in the form of a 4-10 oclock panel diagonal; and wherein the refracting of said light by said positioning utilizes the unmodified plano portion of said planoconcave lens for effecting light radiation refraction to provide substantially equilateral dot pattern exposure on substantially all areas of said sensitized panel except substantially the peripheral 10 oclock panel area whereon improved light refraction is provided by light incident upon said chordal lens portion oriented relative to that quadrantal area to achieve a substantially equilateral dot pattern thereover; and wherein the refracting of light for exposing a second phosphor dot pattern is provided by a second positioning of said lens and said light source oriented in like previous manner relative to another of said orientation diagonals in the form of a 28 oclock panel diagonal whereof improved light refraction is provided to substantially the peripheral 2 oclock panel area by light incident upon said chordal lens portion to achieve a substantially equilateral dot pattern thereover while light incident upon said unmodified plano lens portion provides light refraction to produce substantially equilateral dot pattern exposure for the remaining panel area.

5. The method of fabricating a screen pattern according to claim 2 whereof said light refracting and exposing steps are accomplished by positioning said lens and said light source relative to one another and to said central axis and one of said orientation diagonals in the form of a 4-l0 oclock panel diagonal, and whereof said lens has a modified plano surface oriented substantially opposite said nonsymmetrical spherical concavity, said modified surface comprising a first modified portion and a second modified portion with an intervening substantially unmodified plano portion therebetween, said second chordal portion being substantially diametrically opposed to said first chordal portion and shaped as a second substantially cylindrical section whereof the axis is substantially parallel to the axis of said first substantially cylindrical section, said second chordal portion being tangent to said unmodified plano surface at a second substantially linear region of demarcation to impart a slight opposed concave curvature to said second chordal surface in a direction away from said spherical concavity and effect a gradual increase in the refractive thickness of said lens in a second selected peripheral area thereof; and whereof the positioning of said lens provides light refraction to effect substantially equilateral phosphor dot pattern exposure over substantially the whole area of said sensitized panel, the peripheral 10 oclock panel area having light refraction provided substantially thereto by light incident upon said first chordal lens portion positioned relative to that quadrantal area and the peripheral 4 o'clock panel area having light refraction substantially provided by light incident upon said second chordal lens portion positioned relative to that quadrantal area while light incident upon said unmodified plano lens portion provides light refraction to produce substantially equilateral clot pattern exposure for the remaining panel area; and wherein the refracting of light for exposing a second phosphor dot pattern is provided by a second positioning of said lens and said light source oriented in like previous manner relative to another of said orientation diagonals in the form of a 28 o'clock panel diagonal whereby light refraction is provided to substantially the peripheral 2 oclock panel area by light incident upon said first chordal lens portion positioned relative to that quadrantal area and the peripheral 8 oclock panel area having light refraction substantially provided by light incident upon said second chordal lens portion positioned relative to that quadrantal area, said refraction of light incident upon said unmodified intervening plano lens portion providing substantially equilateral dot pattern exposure for the remaining panel area. 

1. A method of photofabricating at least one of a plurality of triadically related dot patterns forming the cathodoluminescent screen for a color cathode ray tube having a central axis, a screen-bearing panel, and a system of electron guns emanating plural electron beams capable of being deflected from static paths to scan said screen through an intermediately positioned apertured mask spaced adjacent to said screen, said central axis and the axis of said panel and said electron gun system being substantially coincidental, said method being consummated with reference to One of three equispaced orientation diagonals intersecting said central axis and traversing said panel, said method comprising the steps of: applying a coating of a light sensitive substance and an electron responsive phosphor material to said screen-bearing panel; orienting said mask in spaced adjacency to said coated panel to provide a mated mask-panel assembly; positioning said mated assembly relative to a point source of light radiation having an axis laterally removed from said central axis, said light source being oriented relative to the subsequent positioning of one of said electron guns to discretely direct light radiation through said apertures of said mask to said sensitized panel; positioning a refractive medium in the form of a modified planoconcave lens in a predetermined manner intermediate said light source and said mask with said concave surface being oriented toward said mask, said lens having a plane of symmetry containing a quasi-axis of symmetry and a lateral axis of placement perpendicularly intersecting said quasi-axis and said plane of symmetry, said modified portion of said plano surface being at least one substantially chordal portion thereof bisected by said plane of symmetry, said lens positioning being in a manner wherein said plane of symmetry contains said light source axis and said central axis with said quasi-axis being removed from said central axis therein; refracting said light radiation from said source through said lens in a manner to effect directional irradiation to said masked panel to provide a locus of motion of apparent light beam origin that is substantially coincident with the locus of motion of the apparent center of said subsequent electron beam deflection, a portion of said light incident upon said lens being refracted by said modified chordal portion whereof said light rays incident thereupon are directively refracted to a substantially quadrantal area of said mask; exposing said coated panel through said mask to provide the selective imprinting of one of said plural screen patterns on said light sensitive substance; and developing said selectively exposed panel to remove the unexposed material therefrom to provide one discrete dot pattern of said screen.
 2. The method of fabricating a screen pattern according to claim 1 whereof said light refracting and exposing steps are dependent upon the positioning of said lens wherein said concave surface thereof being formed as a spherical concavity is positioned relative to said central and said light source axes in a manner that the concave lens portion substantially utilized in screen exposure is a section eccentrically removed from but containing said quasi-axis and defined as a nonsymmetrical spherical concavity, and wherein the positioning of said lens orients a first chordal portion thereof on the opposite side of said central axis from said quasi-axis to refract that portion of said light radiation incident thereon to substantially the adjacent quadrantal portion of said panel in a manner to move the light ray landings radially inward and provide the disposure of a substantially equilateral phosphor dot pattern thereover, said first chordal lens portion being shaped as a first substantially cylindrical section with the surface thereof tangent to said plano surface at a first substantially linear region of demarcation to impart a slight convex curvature to said modified surface in a direction toward said spherical concavity and effect a gradual reduction in the refractive thickness of said lens in a first selected substantially peripheral area thereof.
 3. A method of fabricating a dot screen pattern according to claim 2 wherein the refracting of said light by the positioning of said modified lens is further defined by optimizing the angles of incidence and refraction thereof by tilting said lens on said lateral axis of placement relative to said central axis.
 4. The method of fabricating a screen pattern according to claim 2 whereof sAid light refracting and exposing steps are accomplished by positioning said lens and said light source relative to one another and to said central axis and one of said orientation diagonals in the form of a 4-10 o''clock panel diagonal; and wherein the refracting of said light by said positioning utilizes the unmodified plano portion of said plano-concave lens for effecting light radiation refraction to provide substantially equilateral dot pattern exposure on substantially all areas of said sensitized panel except substantially the peripheral 10 o''clock panel area whereon improved light refraction is provided by light incident upon said chordal lens portion oriented relative to that quadrantal area to achieve a substantially equilateral dot pattern thereover; and wherein the refracting of light for exposing a second phosphor dot pattern is provided by a second positioning of said lens and said light source oriented in like previous manner relative to another of said orientation diagonals in the form of a 2-8 o''clock panel diagonal whereof improved light refraction is provided to substantially the peripheral 2 o''clock panel area by light incident upon said chordal lens portion to achieve a substantially equilateral dot pattern thereover while light incident upon said unmodified plano lens portion provides light refraction to produce substantially equilateral dot pattern exposure for the remaining panel area.
 5. The method of fabricating a screen pattern according to claim 2 whereof said light refracting and exposing steps are accomplished by positioning said lens and said light source relative to one another and to said central axis and one of said orientation diagonals in the form of a 4-10 o''clock panel diagonal, and whereof said lens has a modified plano surface oriented substantially opposite said nonsymmetrical spherical concavity, said modified surface comprising a first modified portion and a second modified portion with an intervening substantially unmodified plano portion therebetween, said second chordal portion being substantially diametrically opposed to said first chordal portion and shaped as a second substantially cylindrical section whereof the axis is substantially parallel to the axis of said first substantially cylindrical section, said second chordal portion being tangent to said unmodified plano surface at a second substantially linear region of demarcation to impart a slight opposed concave curvature to said second chordal surface in a direction away from said spherical concavity and effect a gradual increase in the refractive thickness of said lens in a second selected peripheral area thereof; and whereof the positioning of said lens provides light refraction to effect substantially equilateral phosphor dot pattern exposure over substantially the whole area of said sensitized panel, the peripheral 10 o''clock panel area having light refraction provided substantially thereto by light incident upon said first chordal lens portion positioned relative to that quadrantal area and the peripheral 4 o''clock panel area having light refraction substantially provided by light incident upon said second chordal lens portion positioned relative to that quadrantal area while light incident upon said unmodified plano lens portion provides light refraction to produce substantially equilateral dot pattern exposure for the remaining panel area; and wherein the refracting of light for exposing a second phosphor dot pattern is provided by a second positioning of said lens and said light source oriented in like previous manner relative to another of said orientation diagonals in the form of a 2-8 o''clock panel diagonal whereby light refraction is provided to substantially the peripheral 2 o''clock panel area by light incident upon said first chordal lens portion positioned relative to that quadrantal area and the peripheral 8 o''clock panel area having light refraction substantially provideD by light incident upon said second chordal lens portion positioned relative to that quadrantal area, said refraction of light incident upon said unmodified intervening plano lens portion providing substantially equilateral dot pattern exposure for the remaining panel area. 